Cilia and flagella are ideal cell organelles for investigation of protein biochemical-structural-functional relationships in motile systems. Moreover, their main structural component, microtubules, are common to many intracellular organelles of motility, sensory transduction, or architecture. Studies on the conformation of microtubule protein or tubulin which forms the 13 protofilaments of single microtubules will be extended, primarily by continued application of biochemical and high resolution electron microscopic techniques. Of particular concern is the structural identification of both the colchicine-binding tubulin dimer of molecular weight approximately 115,000 daltons and the tubulin monomer of molecular weight approximately 55,000 daltons. Importantly, the nature of their incorporation into protofilaments may be related to the specific attachment sites of secondary structures such as radial links and dynein (ATPase) arms to the microtubules. The geometrical arrangement of the radial links connecting the doublet microtubules with the projections of the central sheath is directly related to the motile process. The attachment sites of the linkages and their possible function as a feedback mechanism in motility will be studied by both high resolution electron microscopy and polypeptide characterization. The radial link head-central sheath attachment site may be a region of active displacement in order to both accommodate and regulate doublet microtubule sliding. It is probable that all motile cilia and flagella have a reasonably similar molecular mechanism of movement: microtubule sliding generated by an active crossbridge mechanism between adjacent doublet tubules and bending mediated by prominent radial connections. The function of the linkages and the nature of the crossbridge mechanism will undoubtedly be shown to be intimately related to the polypeptide composition and architecture of microtubules.